As light-transmitting materials, so far, materials transparent to visible light, such as transparent resin, glass, quartz and light-transmitting alumina have been selectively used depending upon the limitations such as an environment in which they are used, cost and the like. For instance, a transparent resin or a glass is used as a cover (window material) for a light source having a low energy intensity and as a light-transmitting material for transmitting light of wavelengths without almost containing ultraviolet rays. As a light-transmitting cover used for a light source that contains some amounts of ultraviolet rays or a light source which has a large energy intensity and of which the temperature becomes high, there is used a material comprising quartz or alumina. As a light-transmitting cover used for a light source that uses a corrosive gas such as halogen gas, further, there is used an alumina material having a large resistance against corrosion.
However, limitation is often imposed on the material used as the light-transmitting material due to an increase in the amount of heat accompanying an increase in the brightness of the light source. For example, the alumina material has resistance against the halogen gas. Concerning the radiation of heat, however, it has a thermal conductivity of as small as 30 W/mK and cannot fully radiate the heat from the light source which, therefore, may shorten the life of the members of the light source.
From the above point of view, an aluminum nitride sintered body is drawing attention as a light-transmitting material because of its favorable heat resistance and heat conducting property as well as excellent corrosion resistance against the halogen gas. However, this sintered body has a lower light transmission factor than those of other light-transmitting materials. Therefore, a variety of reformed aluminum nitride sintered bodies having improved light-transmitting property have been proposed.
For example, a patent document 1 proposes a method of producing an aluminum nitride sintered body by using, as a starting material, an aluminum nitride powder having a fine particle diameter and containing little metal impurities, and firing the aluminum nitride powder in an inert atmosphere at 1700 to 2100°C. The aluminum nitride sintered body obtained by this method exhibits a light transmission factor of 60 to 75% over a wavelength range of 0.2 μm to 30 μm.
Further, a patent document 2 discloses an aluminum nitride sintered body having an oxygen concentration of not more than 400 ppm, a concentration of metal impurities of not more than 150 ppm and a carbon concentration which is suppressed to be not more than 200 ppm, and an average crystal particle diameter of 2 μm to 20 μm. With this sintered body, the gradient of a spectral curve is not less than 1.0 (%/nm) with a thickness of 0.3 mm over a wavelength region of 260 to 300 nm, a total light transmission factor is not less than 86% over a wavelength region of 400 to 800 nm, and a wavelength with which the total light transmission factor reaches 60% in the spectrum is not longer than 400 nm.    Patent document 1: JP-A-2-26871    Patent document 2: JP-A-2005-119953